1. Field of the Invention
The invention relates to a flexible layer composite comprising a support and at least one layer of curable surface coating composition applied to the support and also uses of such a layer composite. The invention further relates to three-dimensional shaped bodies coated with this layer composite and also the surface coating composition which is applied to the support.
2. Background Art
Conventional surface coatings, in particular conventional motor vehicle coatings, usually consist of a plurality of superposed layers of surface coating compositions. However, due to the process, it is frequently difficult to achieve precisely color-matched coatings, e.g. on various substrates such as sheet metal or plastic. Different color shades also result on identical substrates due to even minimal changes in the surface coating components used or a process step.
To solve these problems, multilayer composites comprising a layer of curable surface coating composition applied to a support can be employed in place of conventional surface coatings to coat the respective substrates. In particular, surface coating films in which the support is a film may be mentioned here. The application of these surface coating films forms a coating having a substrate- and process-independent color shade, which considerably simplifies color quality management, for example in automobile construction where finished, coated individual components are usually procured from different suppliers. A further advantage of the use of surface coating films is that no solvent emissions, or comparatively low solvent emissions, occur when they are applied.
WO 00/13893 discloses surface coating films coated with a plurality of layers, with the surface of a plastic film A being coated with at least one surface coating layer and the surface coating layer optionally being coated with a further thermoplastic film B. The plastic film A may, if desired, be able to be pulled off from the surface coating layer which has been applied to a substrate and cured, so that only the surface coating layer remains on the substrate. The surface coating composition used comprises a binder based on polyester, polyurethane or polyacrylate and a crosslinker (an amino resin, a polyisocyanate resin or a carboxyl-containing crosslinker).
EP 361 351 A1 discloses a process according to which a surface coating film comprising a support film, an adhesive layer and a surface coating layer can be applied to three-dimensional curved surfaces. To protect against contamination, the radiation-curable surface coating layer of the surface coating film is partially cured by means of radiation so that it is dust-dry. The surface coating film is subsequently heated and applied. After application, the surface coating layer is subjected to final curing by thermal means.
EP 0 819 520 A2 claims a process for applying a surface coating film comprising at least one surface coating layer and an adhesive layer to three-dimensional curved surfaces, in which the radiation-curable surface coating layer of the surface coating film is partially cured so as to be dust-dry before application to the curved surface and it is subjected to final curing after application. Here, the final curing of the surface coating layer is effected by means of electromagnetic radiation having a wavelength shorter than the wavelength of the visible spectrum. To apply the surface coating film to the substrate, the surface coating layer is preferably heated to its glass transition temperature Tg or slightly above this in order to be able to apply the surface coating film without bubbles, creases and an “orange skin” surface. A surface coating having a glass transition temperature Tg of less than 40° C., in particular less than 30° C., is therefore chosen. The surface coating comprises a binder based on phospacene, urethane or acrylate.
A fundamental problem which occurs in these processes is the susceptibility of the surface coating films used to contamination, in particular by means of dust. Weeks to months can elapse between the time when they are produced until they are processed. If the surface coating composition comes into contact with contaminants during this time, this frequently leads to undesirable optical changes in the surface of the coating. Contaminants of all types have to be avoided at all costs for this reason. In industrial practice, this means that the cleanliness of storage and production buildings has to meet exacting requirements.
One possible measure for reducing these problems is to provide the surface coating layer of a surface coating film with a protective covering or film during or immediately after it has been produced.
Thus, DE 101 40 769 A1 discloses surface coating films in which a polypropylene film is provided with a layer of a clear coating onto which a colored surface coating layer has in turn been applied. To ensure that the surface coating film can be removed from the substrate again without leaving residues, an adhesive layer was applied to the colored surface coating layer and a reinforcing film was in turn laminated on top of this. A release film was laminated onto the reinforcing film to cover it until processing. To ensure sufficient adhesion of the release film to the composite, the use of a further adhesive layer between release film and reinforcing film is necessary.
In general, use of protective films whose use is associated with the use of adhesives is not optimal from a number of points of view. When the protective film is removed, it may be desirable, in particular, for the underlying adhesive layer to be able to be removed as well without leaving residues. In practice, difficulties can occur here. In addition, application of a protective film by means of adhesive to a surface coating layer to be protected requires an additional process step.
Surface coatings having high Tg values are in principle insensitive to contamination, and protection of the surface coating is then not absolutely necessary.
DE 199 17 965 describes a radiation-curable composite layer plate or film comprising at least one substrate layer and a covering layer, with the covering layer comprising a radiation-curable composition containing a binder having a glass transition temperature Tg of above 40° C., preferably above 50° C., in particular above 60° C. The application of a protective layer is accordingly mentioned as a purely optional measure.
However, the use of surface coatings which contain binders having a high glass transition temperature Tg also has disadvantages. Application of a surface coating film is preferably carried out at elevated temperatures, in particular at temperatures above the glass transition temperature Tg of the radiation-curable binder. Tg values which are too high result in poor handling of the physically dried film of surface coating at room temperature. The surface coating is then too brittle; in particular, it is not able to undergo deep drawing and has a tendency to crack formation.